This invention relates to devices for measuring the level of radon gas in air, soil or water.
There are several detectors used to measure airborne radon which rely upon detecting the .alpha.-particles, .alpha.-particles and/or .gamma.-rays produced by the radioactive decay of radon, Rn-222, and its daughter or progeny isotopes Po-218, Pb-214, Bi-214, and Po-214. One aspect of the present invention is concerned with the design of detectors known as .alpha.-track detectors in which .alpha.-particles from Rn-222, Po-218 and Po-214 decays strike a sensitive detection surface such as polycarbonate plastic or nitrocellulose (see, Likes et al., 159 Nuclear Instruments and Methods 395, 1979). The number of submicroscopic damage tracks produced by these .alpha.-particles per unit area of detection surface per unit of exposure time, provides a measure of radon in the air.
In air, the maximum range of .alpha.-particles produced from radon decay is about 4-5 cm. Thus, for an .alpha.-track detector operating in air, .alpha.-particles which originate within the detector's 3-5 cm diameter air space may register on the detection surface.
The .alpha.-track device is useful largely because it can integrate radon decay events over long periods of time and thus provide valid average radiation exposure levels. The device, however, has been criticized because it is less sensitive than many other measurement techniques, requiring relatively long exposure intervals for accurate measurement of low levels of radon. For example, a CR39 polycarbonate plastic surface in a typical detector should receive about 50 picoCurie/liter-days of radon exposure for accurate radon measurement. Thus, a 25 day exposure period is required to accurately measure an airspace containing radon at a level of 2 pCi per liter of air.
The device is also criticized because it provides a measure of the concentration of daughter decay products generated from radon entering the detector, rather than radon itself. This criticism is not valid if the ratio of .alpha.-particles detected from radon daughters divided by the radon gas concentration is constant. Many environmental factors, however, during radon testing are difficult to control. Some of these factors are known to affect this ratio and thus compromise the accuracy of the test. For example, static electricity or dust, on or near a detector, can influence the local depositing of the electrically charged radon daughters. If the radon daughters inside one detector remain largely airborne and diffuse randomly while the daughters in another detector are deposited on the sidewalls, the frequency of their emitted .alpha.-particles striking the single detection surface in each device will be different.
To overcome the relatively low sensitivity of the .alpha.-track detector, several strategies have been utilized to increase the .alpha.-particle track density for a given radon level and exposure interval. One detector, manufactured by Tech/Ops Landauer, Inc., employs a nitrocellulose membrane which is exposed directly to ambient air to maximize the incident flux of .alpha.-particles from radon daughters. Although nitrocellulose is a very sensitive detection material for .alpha.-particles, it is more susceptible than polycarbonate plastic to the effect of moisture which can unpredictably alter the efficiency of the material in registering incident .alpha.-particles (Likes et al., supra).
A second design concept to increase .alpha.-track detector sensitivity has been recently reported (Miyake et al., 26 No. 4, Japanese Journal of Applied Physics 607, 1987; and Kotrappa et al., 43 Health Physics 399, 1982). This involves placing an electrically negative charged surface in proximity to the .alpha.-track detection surface to amplify the .alpha.-particle flux from positively charged radon daughters. The electrically charged surface has itself been utilized in the so-called "Electret" commercial detector device to quantitate radon concentration (Khan et al., 46, No. 1, Health Physics 141, 1984; Kotrappa et al., supra). Radon levels are measured by accumulating the daughter ions on an electrostatically negative charged TEFLON.RTM. surface and precisely measuring the rate of voltage drop over the exposure interval. If, as commonly occurs, an Electret surface receives an uneven deposit of radon daughters and is positioned next to an .alpha.-track detection surface, the .alpha.-track surface will, in turn, receive an uneven pattern of irradiation by the .alpha.-particles produced from the deposited daughters. Such an uneven pattern makes analysis of the .alpha.-track density in the detection surface difficult and contributes uncertainty to the measured level of radon. Furthermore, the charged surface accumulates diffusible radon daughters, not radon gas, leading to the same uncertainties in radon measurement described above for the unmodified .alpha.-track detector.